Image coding method and apparatus using spatial predictive coding of chrominance and image decoding method and apparatus

ABSTRACT

A coding method including dividing pixels of a chrominance component of an input image into blocks having a predetermined size; selecting one among a direct current prediction method, a vertical prediction method, a horizontal prediction method, and a hybrid prediction method according to a user&#39;s input; generating a prediction value of each pixel in a current block to be predictively coded, using at least one pixel value among pixel values in an upper reference block adjacent to the current block and in a side reference block adjacent to the current block, according to the selected prediction method; generating a differential value between the prediction value and a corresponding real pixel value in the current block; and coding the differential value and information on the selected prediction method using a predetermined coding method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/829,073 filed Dec. 1, 2017, which is a continuation of U.S.application Ser. No. 15/388,205, filed Dec. 22, 2016, now U.S. Pat. No.9,866,842, which is a continuation of U.S. application Ser. No.15/003,275, filed Jan. 21, 2016, now U.S. Pat. No. 9,565,448, which is acontinuation of U.S. application Ser. No. 14/456,388, filed Aug. 11,2014, now U.S. Pat. No. 9,282,341, which is a continuation of U.S.application Ser. No. 13/673,331, filed Nov. 9, 2012, abandoned, which isa divisional of U.S. application Ser. No. 11/882,869, filed Aug. 6,2007, now U.S. Pat. No. 8,345,995, which is a divisional of U.S.application Ser. No. 10/673,186, filed Sep. 30, 2003, now U.S. Pat. No.7,266,247, which claims the priority of Korean Patent Application No.10-2002-0059468, filed Sep. 30, 2002, and Korean Patent Application No.10-2003-0055887, filed Aug. 12, 2003, in the Korean IntellectualProperty Office, the disclosures of which are incorporated herein byreference.

BACKGROUND 1. Field

The present invention relates to image coding and decoding, and moreparticularly, to a method and apparatus for coding a chrominancecomponent of an intra-image using spatial predictive coding and a methodand apparatus for decoding the coded chrominance component.

2. Description of the Related Art

When an image or a motion image is compressed, the image is usuallydivided into a luminance component and a chrominance component, whichare coded. The luminance component and the chrominance component havedifferent statistical characteristics. Since human eyes are moresensitive to a change in the luminance component than to a change in thechrominance component, a sampling frequency for the luminance componentis usually two or four times higher than that for the chrominancecomponent. Pixel values of the chrominance component have a lessvariance than pixel values of the luminance component.

In conventional international standard technology for compressing amotion image, a single image is divided into a chrominance component anda luminance component and then coded. The image is coded withoutreferring to another image. The coded image is referred to when imagestemporally following the coded image are predictively coded using motionestimation and compensation. The image coded without referring toanother image is referred to as an intra-image, and the image codedusing motion estimation and compensation referring to another image isreferred to as an inter-image. The intra-image and the inter-image arelossy compressed through discrete cosine transformation (DCT),quantization, and entropy coding. Here, since temporal prediction is notused for the intra-image, spatial prediction is used for the intra-imageto increase compression efficiency.

In motion image compression technology according to InternationalOrganization for Standardization/International ElectrotechnicalCommission (ISO/IEC) Motion Picture Experts Group (MPEG)-4 andInternational Telecommunication Union-Telecommunication Standardization(ITU-T) H.263+, when a spatial prediction is performed on theintra-image, an 8×8 pixel block is defined, and DCT and quantization areperformed on each block. Next, direct current (DC) values andalternating current (AC) values of a current block are predictivelycoded referring to DC values and AC values of adjacent blocks toincrease compression efficiency.

Recently, ISO/IEC MPEG and ITU-T Video Coding Experts Group (VCEG)organized a joint video team (JVT) to develop a new video codingstandard. The final recommendation of the JVT committee includestechnology for compressing an intra-image using spatial predictivecoding. In this technology, a block size and a spatial prediction methodused for a luminance component are different from those used for achrominance component. A block of 4×4 or 16×16 is used for the luminancecomponent. When a 4×4 block is used, 9 prediction methods are usedaccording to a prediction direction. When a 16×16 block is used, 4prediction methods are used according to a prediction direction.

Similarly to prediction using a 16×16 block for the luminance component,prediction for the chrominance component uses 4 prediction methods inwhich a block has a size of 8×8. In FIG. 1A, “q” denotes a value of eachpixel in a current block of 8×8 to be coded or a value of a pixel in ablock adjacent to the current block. A pixel value in the adjacent blockis used to predict a pixel value in the current block. Specifically, aDC prediction method, a vertical prediction method, a horizontalprediction method, and a plane prediction method are used. In eachprediction method, before pixel values in the current block are beingcoded, values of the respective pixels in the current block arepredicted referring to values of pixels at the edges in adjacent blocks.The edges of the adjacent blocks respectively meet the left and the topof the current block. Next, a differential value between a predictedvalue, i.e., a prediction value of each pixel in the current block and acorresponding real pixel value in the current block is coded.

The DC prediction method uses an average of pixel values referred to.Referring to FIG. 1B, S0 denotes an average of pixel values q₁₀, q₂₀,q₃₀, and q₄₀. S1 denotes an average of pixel values q₅₀, q₆₀, q₇₀, andq₈₀. S2 denotes an average of pixel values q₀₁, q₀₂, q₀₃, and q₀₄. S3denotes an average of pixel values q₀₅, q₀₆, q₀₇, and q₀₈. A pixel valuein a block A of 4×4 is predicted using the averages S0 and S2. If onlyone of the averages S0 and S2 can be referred to, prediction isperformed using the average S0 or S2 that can be referred to. If neitherof the averages S0 and S2 can be referred to, a value of 128 is used forprediction. A pixel value in a block B of 4×4 is predicted using theaverage S1. If the average S1 cannot be referred to, the average S2 isreferred to. If even the average S2 cannot be referred to, a value of128 is used for prediction. A pixel value in a block C of 4×4 ispredicted using the average S3. If the average S3 cannot be referred to,the average S0 is referred to. If even the average S0 cannot be referredto, a value of 128 is used for prediction. A pixel value in a block D of4×4 is predicted using the averages S1 and S3. If only one of theaverages S1 and S3 can be referred to, prediction is performed using theaverage S1 or S3 that can be referred to. If neither of the averages S1and S3 can be referred to, a value of 128 is used for prediction.

In performing predictive coding, a differential value “p_(xy)′” obtainedby subtracting a prediction value “pred” generated using a pixel valuein an adjacent block from a corresponding pixel value “p_(xy)” in acurrent block to be coded is coded. For example, when all of theaverages S0 through S3 can be used, the differential value “p_(xy)′” tobe coded using frequency transformation and quantization and theprediction value “pred” depending on a coordinate value of the pixel aredefined by Formula (1).

p _(xy) ′=p _(xy)−pred,

pred=(S0+S2)/2, 1≤x,y≤4,

pred=S1, 5≤x≤8, 1≤y≤4,

pred=S3, 1≤x≤4, 5≤y≤8,

pred=(S1+S3)/2, 5≤x,y≤8   (1)

Meanwhile, in the vertical prediction method, predictive coding isperformed in a vertical direction using a value of a pixel above acurrent block. In other words, pixels on the same column have the sameprediction value q.sub.x0, and a differential value to be coded isgenerated using Formula (2).

p _(xy) ′=p _(xy) −q _(x0), 1≤x,y≤8   (2)

In the horizontal prediction method, predictive coding is performed in ahorizontal direction using a value of a pixel on the left of a currentblock. In other words, pixels on the same row have the same predictionvalue q.sub.0y, and a differential value to be coded is generated usingFormula (3).

p _(xy) ′=p _(xy) −q _(0y), 1≤x,y≤8   (3)

In the plane prediction method, a vertical variation and a horizontalvariation are obtained using pixel values referred to, and pixel valuesin a current block are predicted according to a plane equation using thevertical and horizontal variations and the pixel values referred to. Inother words, when a prediction value for a pixel value “p_(xy)” in acurrent block is denoted by “pred_(xy)”, the prediction value“pred_(xy)” and a differential value “p_(xy)′” are generated usingFormula (4).

$\begin{matrix}{{p_{xy}^{\prime} = {p_{xy} - {pred}_{xy}^{\prime}}},{{pred}_{xy} = {\left( {a + {b \times \left( {x - 3} \right)} + {c \times \left( {y - 3} \right)}} \right)/32}},{a = {16 \times \left( {q_{80} + q_{08}} \right)}},{b = {\left( {17 \times {dH}} \right)/32}},{c = {\left( {17 \times {dV}} \right)/32}},{{dH} = {{\sum\limits_{x^{\prime} = 1}^{4}\; {x^{\prime} \times \left( {q_{{4 + x^{\prime}},0} - q_{{4 - x^{\prime}},0}} \right){dV}}} = {\sum\limits_{y^{\prime} = 1}^{4}\; {y^{\prime} \times \left( {q_{0,{4 + y^{\prime}}} - q_{0,{4 - y^{\prime}}}} \right)}}}}} & (4)\end{matrix}$

Here, dH and dV denote the horizontal variation and the verticalvariation, respectively.

The plane prediction method is disadvantageous in that a large amount ofcalculation is required because the vertical and horizontal variationsneed to be calculated and a prediction value of each pixel needs to becalculated using the plane equation.

In order to indicate which of the four prediction methods has been usedduring coding, entropy coding is performed using a variable-length codeso that compensation during decoding is performed using the predictionmethod used during coding.

SUMMARY

The present invention provides a coding and decoding method forperforming effective prediction with a small amount of calculationtaking account of a statistical characteristic of a chrominancecomponent when performing spatial predictive coding of the chrominancecomponent in an intra-image, and an apparatus therefor.

The present invention also provides a recording medium for storing aprogram code for executing the above-described coding and decodingmethod in a computer.

According to an aspect of the present invention, there is provided acoding apparatus including a variation calculator, which calculates avertical variation and a horizontal variation with respect to a currentblock to be predictively coded among blocks having a predetermined size,into which a chrominance component of an input image is divided, usingpixel values in an upper reference block adjacent to the current blockand pixel values in a side reference block adjacent to the currentblock; a hybrid predictor, which divides the current block into apredetermined number of regions according to the vertical and horizontalvariations and generates a prediction value of each pixel in each regionusing a pixel value in the upper reference block or a pixel value in theside reference block; a differential value generator, which generates adifferential value between the prediction value and a corresponding realpixel value in the current block and codes the differential value usinga predetermined coding method.

According to another aspect of the present invention, there is provideda coding apparatus including a hybrid predictor, which divides a currentblock to be predictively coded among blocks having a predetermined size,into which a chrominance component of an input image is divided, into apredetermined number of regions according to a predetermined number ofprediction methods and generates prediction values of each pixel in thecurrent block according to the respective prediction methods using apixel value in an upper reference block adjacent to the current blockand a pixel value in a side reference block adjacent to the currentblock; a differential value generator, which generates differentialvalues between the prediction values corresponding to the respectiveprediction methods and a corresponding real pixel value in the currentblock; a selector, which selects a differential value requiring a leastnumber of bits for coding among the differential values; and a coder,which codes the selected differential value and information on aprediction method corresponding to the selected differential value usinga predetermined coding method.

According to still another aspect of the present invention, there isprovided a coding apparatus including a selector, which selects oneamong predetermined prediction methods comprising a direct currentprediction method, a vertical prediction method, a horizontal predictionmethod, and a hybrid prediction method according to a user's input; apredictor, which generates a prediction value of each pixel in a currentblock to be predictively coded among blocks having a predetermined size,into which a chrominance component of an input image is divided, usingat least one pixel value among pixel values in an upper reference blockabove the current block and in a side reference block on left of thecurrent block, according to the selected prediction method; adifferential value generator, which generates a differential valuebetween the prediction value and a corresponding real pixel value in thecurrent block; and a coder, which codes the differential value andinformation on the selected prediction method using a predeterminedcoding method.

Preferably, the predictor includes a hybrid predictor, and the hybridpredictor calculates a vertical variation and a horizontal variationwith respect to the current block using pixel values adjacent to thecurrent block in the upper and side reference blocks, divides thecurrent block into a predetermined number of regions according to thevertical and horizontal variations, and generates prediction values ofrespective pixels in each region using the pixel values in the upper andside reference blocks.

According to still another aspect of the present invention, there isprovided an apparatus for decoding a bitstream resulting from coding achrominance component of an image to restore the image. The apparatusincludes a decoder, which decodes each differential value for thechrominance component included in the bitstream in units of blocks usinga predetermined decoding method corresponding to coding information readfrom the bitstream; a prediction method determiner, which determineswhether a prediction mode indicating information on a prediction methodis included in the bitstream, extracts the prediction mode from thebitstream when the prediction mode is determined as being included inthe bitstream, determines the prediction method based on the extractedprediction mode, calculates a vertical variation and a horizontalvariation with respect to a current block to be restored using pixelvalues in an upper reference block and a side reference block, whichhave been restored prior to the current block, when the prediction modeis determined as not being included in the bitstream, and determines theprediction method according to the vertical and horizontal variations; aprediction value generator, which generates a prediction value of eachpixel in the current block according to the determined predictionmethod; and a predictive compensator, which adds the prediction value toa corresponding differential value to restore the chrominance componentof the image.

Preferably, when the prediction method is determined according to thevertical and horizontal variations, the prediction value generatorcompares the vertical variation with the horizontal variation, dividesthe current block into a plurality of regions in a predetermineddirection according to the result of comparison, and generatesprediction values of respective pixels in each region using pixel valuesin the upper and side reference blocks.

According to still another aspect of the present invention, there isprovided a coding method including dividing pixels of a chrominancecomponent of an input image into blocks having a predetermined size;generating a vertical variation and a horizontal variation with respectto a current block to be predictively coded, using pixel values in anupper reference block adjacent to the current block and pixel values ina side reference block adjacent to the current block; dividing thecurrent block into a predetermined number of regions according to thevertical and horizontal variations and generating a prediction value ofeach pixel in each region using a pixel value in the upper referenceblock or a pixel value in the side reference block; and generating adifferential value between the prediction value and a corresponding realpixel value in the current block and coding the differential value usinga predetermined coding method.

According to still another aspect of the present invention, there isprovided a coding method including dividing pixels of a chrominancecomponent of an input image into blocks having a predetermined size;dividing a current block to be predictively coded into a predeterminednumber of regions according to a predetermined number of predictionmethods and generating prediction values of each pixel in the currentblock according to the respective prediction methods using a pixel valuein an upper reference block adjacent to the current block and a pixelvalue in a side reference block adjacent to the current block;generating differential values between the prediction valuescorresponding to the respective prediction methods and a correspondingreal pixel value in the current block; and selecting a differentialvalue requiring a least number of bits for coding among the differentialvalues and coding the selected differential value and information on aprediction method corresponding to the selected differential value usinga predetermined coding method.

According to still another aspect of the present invention, there isprovided a coding method including dividing pixels of a chrominancecomponent of an input image into blocks having a predetermined size;selecting one among a direct current prediction method, a verticalprediction method, a horizontal prediction method, and a hybridprediction method according to a user's input; generating a predictionvalue of each pixel in a current block to be predictively coded, usingat least one pixel value among pixel values in an upper reference blockadjacent to the current block and in a side reference block adjacent tothe current block, according to the selected prediction method;generating a differential value between the prediction value and acorresponding real pixel value in the current block; and coding thedifferential value and information on the selected prediction methodusing a predetermined coding method.

Preferably, the hybrid prediction method includes calculating a verticalvariation and a horizontal variation with respect to the current blockusing pixel values adjacent to the current block in the upper and sidereference blocks, dividing the current block into a predetermined numberof regions according to the vertical and horizontal variations, andgenerating prediction values of respective pixels in each region usingthe pixel values in the upper and side reference blocks.

According to still another aspect of the present invention, there isprovided a method of decoding a bitstream resulting from coding achrominance component of an image to restore the image. The methodincludes (a) decoding each differential value for the chrominancecomponent included in the bitstream in units of blocks using apredetermined decoding method corresponding to coding information readfrom the bitstream; (b) determining whether a prediction mode indicatinginformation on a prediction method is included in the bitstream,extracting the prediction mode from the bitstream, and determining theprediction method based on the extracted prediction mode; (c) when it isdetermined that the prediction mode is not included in the bitstream,calculating a vertical variation and a horizontal variation with respectto a current block to be restored using pixel values in an upperreference block and a side reference block, which have been restoredprior to the current block, and determining the prediction methodaccording to the vertical and horizontal variations; (d) generating aprediction value of each pixel in the current block according to theprediction method determined in step (b) or (c); and (e) adding theprediction value to a corresponding differential value to restore thechrominance component of the image.

Preferably, the prediction method determined in step (c) includescomparing the vertical variation with the horizontal variation, dividingthe current block into a plurality of regions in a predetermineddirection according to the result of comparison, and generatingprediction values of respective pixels in each region using pixel valuesin the upper and side reference blocks.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIGS. 1A and 1B illustrate a conventional spatial prediction method fora chrominance component;

FIG. 2A is a block diagram of an image coding apparatus according to anembodiment of the present invention;

FIG. 2B is a flowchart of an image coding method according to anembodiment of the present invention;

FIGS. 3A through 3D are schematic block diagrams of preferredembodiments of a chrominance predictive coding unit shown in FIG. 2A;

FIGS. 4A through 4D are flowcharts of preferred embodiments of spatialprediction of chrominance shown in FIG. 2B;

FIGS. 5A through 5H illustrate a method of dividing a block into tworegions to perform predictive coding of a chrominance componentaccording to the present invention;

FIG. 6A is a block diagram of an image decoding apparatus according toan embodiment of the present invention;

FIG. 6B is a flowchart of an image decoding method according to anembodiment of the present invention;

FIG. 7A is a block diagram of a chrominance spatial-predictivecompensation unit according to an embodiment of the present invention;

FIG. 7B is a flowchart of spatial-predictive compensation of chrominanceaccording to an embodiment of the present invention; and

FIGS. 8A and 8B are graphs showing the test results of comparing amethod of the present invention and a method suggested by therecommendation of the joint video team (JVT) committee in terms ofcompression efficiency.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an image coding and decoding apparatus and method accordingto preferred embodiments of the present invention will be described indetail with reference to the attached drawings.

FIG. 2A is a block diagram of a coding apparatus according to anembodiment of the present invention. The image coding apparatus includesan input unit 100, a luminance predictive coding unit 200, a chrominancepredictive coding unit 300, a temporal predictive coding unit 400, atransformation/quantization unit 500, and an entropy coding unit 550.

An image coding method and apparatus according to the present inventionwill be described with reference to FIGS. 2A and 2B. When an image (forexample, a motion image) to be coded is input to the input unit 100 inunits of frames (S100), the input unit 100 determines whether the imageis an intra-image or an inter-image and outputs the image to thetemporal predictive coding unit 400 when the image is determined as theinter-image and to the luminance predictive coding unit 200 when theimage is determined as the intra-image (S110).

The luminance predictive coding unit 200 codes a luminance component ateach predetermined block in the intra-image (S200). Here, the luminancepredictive coding unit 200 spatially predicts a pixel value of aluminance component in a current block to be coded using a pixel valuein an adjacent block and generates a differential value between thepredicted pixel value and a corresponding real pixel value of theluminance component in the current block.

The chrominance predictive coding unit 300 spatially predicts a pixelvalue of a chrominance component in the intra-image and generates adifferential value between the predicted pixel value and a correspondingreal pixel value of the chrominance component (S300). A function andoperation of the chrominance predictive coding unit 300 will bedescribed later in detail.

The temporal predictive coding unit 400 receiving the inter-imagetemporally predicts pixel values in the inter-image using an intra-imageor an inter-image input in advance to the current inter-image, generatesa differential value between each predicted pixel value and acorresponding real pixel value in the current inter-image, and outputsthe differential value to the transformation/quantization unit 500(S400).

The transformation/quantization unit 500 receives the spatiallypredicted differential values, i.e., the differential value of theluminance component and the differential value of the chrominancecomponent, and the temporally predicted differential value, transformsthe predicted differential values into values in frequency domain usinga transformation method such as discrete cosine transformation (DCT),quantizes the predicted differential values in the frequency domainusing predetermined quantization bits, and outputs the quantizedpredicted differential values to the entropy coding unit 550 (S500). Theentropy coding unit 550 codes the quantized predicted differentialvalues using entropy coding such as Huffman coding or arithmetic coding(S550).

After describing hybrid prediction used to perform predictive coding ofa chrominance component according to the present invention withreference to FIGS. 5A through 5H, the chrominance predictive coding unit300 and step S300 will be described in detail with reference to FIGS. 3Athrough 3D, which are schematic block diagrams of preferred embodimentsof the chrominance predictive coding unit 300, and FIGS. 4A through 4D,which are flowcharts of preferred embodiments of the chrominance spatialprediction.

FIGS. 5A through 5H illustrate spatial prediction of a chrominancecomponent according to the present invention. In FIGS. 5A through 5H,each of the squares and circles denotes a pixel. A circle-shape pixeldenotes a pixel in a current block, and 8×8 circle-shape pixelsconstitute a single block. Pixel value prediction is performed in each8×8 block. A square-shape pixel denotes a pixel in a block adjacent tothe current block and is used to predict a pixel value in the currentblock. For clarity of the description, a pixel in an adjacent blockabove the current block is colored black, and a pixel in an adjacentblock on the left of the current block is colored white. Values of eightblack square-shape pixels above the current block change from left toright, and a variation of these values is denoted by dH. Values of eightwhite square-shape pixels on the left of the current block changes fromtop to bottom, and a variation of these values is denoted by dV. Achange in a value in the current block can be predicted based on thesevariations dH and dV.

According to a plane prediction method suggested by the recommendationof a joint video team (JVT) committee, a predicted value has a planeshape gradually changing according to the variations dH and dV. However,in an actual image, a change in a value of a chrominance component isnot great, and a change in the value is intermittent unlike in the planeprediction method in which a value changes gradually. While a value ofluminance gradually changes according to intensity of illumination or anangle between an object and light, a value of chrominance changesintermittently because an object has a unique color.

In order to find a region having such an intermittent change in acurrent block, the block can be divided as shown in FIGS. 5A through 5H.Value of black circle-shape pixels are predicted using values of blacksquare-shape pixels above the current block, and values of whitecircle-shape pixels are predicted using values of white square-shapepixels on the left of the current block.

A value of each hatched circle-shape pixel is predicted using a value ofa black square-shape pixel, a value of a white square-shape pixel, or anaverage of the values of the black and white square-shape pixels. Forexample, in FIG. 5B, a value of a hatched circle-shape pixel above theline can be predicted using a value of a black square-shape pixel, and avalue of a hatched circle-shape pixel below the line can be predictedusing a value of a white square-shape pixel. Alternatively, a value of ahatched circle-shape pixel can be predicted using an average of valuesof a black square-shape pixel and a white square-shape pixel,respectively, which correspond a position of the hatched circle-shapepixel. In this situation, methods illustrated in FIGS. 5B and 5H havethe same result, and methods illustrated in FIGS. 5D and 5F have thesame result.

FIGS. 5A through 5H illustrate eight methods of dividing a block. Twoschemes can be considered to determine which of the eight methods touse. In a first scheme, all of the eight methods are used, and thenamong the results of the eight methods, a method having the most optimalresult is used. When the first scheme is used, a prediction error can beminimized. However, it is necessary to embed information indicatingwhich method has been used during coding into a bitstream to be coded sothat the method used during coding can be used during decoding. Sincethe information is coded, the amount of bits to be coded increases.Accordingly, a method that minimizes a prediction error and needs asmall amount of bits when it is coded must be selected in order toachieve optimal compression efficiency.

In a second scheme, a particular one among the eight methods isdetermined using information which can be obtained during decoding,without coding information indicating a method used during coding. Forexample, since values of pixels in blocks adjacent to a current block,i.e., the values of the square-shape pixels, can be obtained duringdecoding, one among the eight methods can be selected using the valuesof the square-shape pixels. Specifically, the variations dH and dV canbe used. When the variation dH is greater than the variation dV, themethod illustrated in FIG. 5A, 5B, or 5H can be used. When the variationdV is greater than the variation dH, the method illustrated in FIG. 5D,5E, or 5F can be used.

Information indicating a method selected among the three methods can beembedded into a bitstream to be coded, as in the first scheme.Alternatively, one among the three methods can be also selected usingthe values of the square-shape pixels. For example, a variation ofvalues of the upper four pixels among the white square-shape pixels anda variation of values of the lower four pixels among the whitesquare-shape pixels are obtained. When the upper variation is greaterthan the lower variation, the method illustrated in FIG. 5B is selected.When the lower variation is greater than the upper variation, the methodillustrated in FIG. 5H is selected. When the upper and lower variationsare almost the same, the method illustrated in FIG. 5A is selected.Similarly, a variation of values of the first four pixels among theblack square-shape pixels and a variation of values of the last fourpixels among the black square-shape pixels are obtained. When thevariation of values of the first four pixels among the blacksquare-shape pixels is less than the variation of values of the lastfour pixels among the black square-shape pixels, the method illustratedin FIG. 5D is selected. When the first variation is greater than thelast variation, the method illustrated in FIG. 5F is selected. When thetwo variations is almost the same, the method illustrated in FIG. 5E isselected.

In addition, a difference between the vertical variation dV and thehorizontal variation dH is compared with a threshold value. When thedifference is not greater than the threshold value, one of the methodsillustrated in FIGS. 5C and 5G is used for prediction. When a differencebetween an average of the values of the black square-shape pixels and anaverage of the values of the white square-shape pixels is great, themethod illustrated in FIG. 5C is used. When the difference between thetwo averages is small, the method illustrated in FIG. 5G is used.

When all of the eight methods are used, a large amount of calculation isrequired. In order to decrease the amount of calculation, the number ofmethods used for prediction may be reduced. For example, only the methodillustrated in FIG. 5C is used without obtaining the variations dH anddV. In another case, the method illustrated in FIG. 5A is used when thevariation dH is greater than the variation dV, and the methodillustrated in FIG. 5E is used when the variation dV is greater than thevariation dH. In still another case, when an average of values of ablack square-shape pixel and a white square-shape pixel is used as avalue of a hatched circle-shape pixel, the methods illustrated in FIGS.5B and 5H have the same result, and the methods illustrated in FIGS. 5Dand 5F have the same result. Accordingly, when the method illustrated inFIG. 5G is excluded, a total of usable methods is reduced to five.

Conversely, when more directions of the line are added or another shapeof the line dividing a block is considered, more methods can be defined.Even in this situation, which of the methods to use can be determinedusing the above-described two schemes.

When a value of a pixel in a current block is predicted using a value ofa black or white square-shape pixel, it is simplest to use a value of awhite or black square-shape pixel on the same column or row as the pixelin the current block. Alternatively, values of pixels on the left andthe right of a white or black square-shape pixel on the same column orrow as the pixel in the current block may be used. According to adirection of the line dividing the current block, a white or blacksquare-shape pixel parallel to the line may be used. Pixels immediatelyadjacent to the current block and pixels adjacent to the pixelsimmediately adjacent to the current block may be used together.

FIGS. 3A and 4A show the chrominance predictive coding unit 300 and thechrominance predictive coding (S300), respectively, according to a firstembodiment of the present invention. The chrominance predictive codingunit 300 according to the first embodiment includes a variationcalculator 302, a hybrid predictor 304, and a differential valuegenerator 306.

When the chrominance component of the intra-image is input to thechrominance predictive coding unit 300, the variation calculator 302calculates a horizontal variation and a vertical variation of pixelvalues in the current block using pixel values in reference blocksadjacent to the current block, as described above, and outputs thevertical and horizontal variations to the hybrid predictor 304 (S302).

The hybrid predictor 304 compares the horizontal variation and thevertical variation to determine a hybrid prediction method, generates aprediction value of each pixel in the current block according to thedetermined hybrid prediction method, and outputs the prediction value tothe differential value generator 306 (S304).

More specifically, the hybrid predictor 304 determines whether adifference between the vertical variation and the horizontal variationis less than a predetermined threshold value. When the differencebetween the two variations is determined as being less than thepredetermined threshold value, prediction is performed using the methodillustrated in FIG. 5C or 5G according to the magnitude of an averagepixel value, as described above. However, when the difference betweenthe two variations is determined as not being less than thepredetermined threshold value, one of the methods illustrated in FIGS.5A, 5B, and 5H is used for prediction if the horizontal variation isgreater than the vertical variation, and one of the methods illustratedin FIGS. 5D, 5E, and 5F is used for prediction if the vertical variationis greater than the horizontal variation, as described above. A schemeof selecting one among three methods has been described above.

The differential value generator 306 subtracts each prediction valuefrom each corresponding real pixel value of the chrominance component inthe intra-image to generate a differential value and outputs thedifferential value to the transformation/quantization unit 500 (S306).

FIGS. 3B and 4B show the chrominance predictive coding unit 300 and thechrominance predictive coding (S300), respectively, according to asecond embodiment of the present invention. The chrominance predictivecoding unit 300 according to the second embodiment includes a hybridpredictor 312, a differential value generator 314, and a selector 316.

The hybrid predictor 312 generates prediction values of each pixel in aninput block of the chrominance component by performing the eight methodsillustrated in FIGS. 5A through 5H or a predetermined number ofprediction methods and outputs the prediction values corresponding tothe respective prediction methods to the differential value generator314 (S312).

The differential value generator 314 subtracts each of the predictionvalues corresponding to the respective prediction methods from acorresponding real pixel value of the chrominance component in theintra-image to generate differential values corresponding to therespective prediction methods, and outputs the differential values tothe selector 316 (S314).

The selector 316 selects a differential value having a least amount ofdata to be coded among the differential values and a prediction methodcorresponding to the selected differential value and outputs theselected differential value and prediction method to thetransformation/quantization unit 500 (S316). The selector 316 can usevarious schemes to select a prediction method and a differential value.In the simplest schemes, a prediction method giving the least sum ofabsolute values of differential values for all pixels in a current blockand a differential value corresponding to the prediction method areselected. The entropy coding unit 550 codes information on the selectedprediction method together with quantized differential values and embedsthe information into an output bitstream.

FIGS. 3C and 4C show the chrominance predictive coding unit 300 and thechrominance predictive coding (S300), respectively, according to a thirdembodiment of the present invention. The chrominance predictive codingunit 300 according to the third embodiment includes a selector 320, adirect current (DC) predictor 332, a vertical predictor 334, ahorizontal predictor 336, a hybrid predictor 338, and a differentialvalue generator 340. The hybrid predictor 338 is implemented by one ofthe hybrid predictors 304 and 312 shown in FIGS. 3A and 3B. The selector320 receives the chrominance component of the intra-image, selects aspatial prediction method to be performed on the chrominance componentamong a DC prediction method, a vertical prediction method, a horizontalprediction method, and a hybrid prediction method, and outputs thechrominance component to a unit corresponding to the selected predictionmethod (S322). The selector 320 may select a prediction method simplyaccording to a value previously set or currently input by a user oraccording to characteristics of an input image.

The DC predictor 332, the vertical predictor 334, the horizontalpredictor 336, or the hybrid predictor 338 receiving the chrominancecomponent from the selector 320 generates a prediction value of eachpixel according to its prediction method and outputs the predictionvalue to the differential value generator 340 (S324). The differentialvalue generator 340 subtracts the prediction value from a correspondingreal pixel value of the chrominance component to generate a differentialvalue and outputs the differential value and information on theprediction method to the transformation/quantization unit 500 (S326).The DC prediction method performed by the DC predictor 332, the verticalprediction method performed by the vertical predictor 334, and thehorizontal prediction method performed by the horizontal predictor 336have been described above. The hybrid prediction method performed by thehybrid predictor 338 has been also described above with reference toFIGS. 5A through 5H.

Accordingly, a bitstream generated according to the third embodimentincludes coded differential values of the chrominance component andinformation on the selected prediction method. In addition, when thehybrid prediction method is selected and the hybrid predictor accordingto the second embodiment is used, information on a hybrid predictionmethod selected from a plurality of hybrid prediction methods is alsoincluded in the bitstream.

FIGS. 3D and 4D show the chrominance predictive coding unit 300 and thechrominance predictive coding (S300), respectively, according to afourth embodiment of the present invention. The chrominance predictivecoding unit 300 according to the fourth embodiment includes a DCpredictor 352, a vertical predictor 354, a horizontal predictor 356, ahybrid predictor 358, a differential value generator 360, and a selector370. The hybrid predictor 358 is implemented by one of the hybridpredictors 304 and 312 shown in FIGS. 3A and 3B. The chrominancecomponent of the intra-image is input to all of the DC predictor 352,the vertical predictor 354, the horizontal predictor 356, and the hybridpredictor 358, each of which generates a prediction value of each pixelusing its prediction method and outputs the prediction value to thedifferential value generator 360 (S332).

The differential value generator 360 subtracts the prediction value fromeach of the predictors 352, 354, 356, and 358 from a corresponding realpixel value of the chrominance component in the intra-image to generatedifferential values corresponding to the respective prediction methodsand outputs the differential values to the selector 370 (S334). Theselector 370 outputs a differential value having a least amount of datato be coded among the differential values and a prediction methodcorresponding to the selected differential value to thetransformation/quantization unit 500 (S336). The selector 370 may usethe selection scheme used by the selector 316 shown in FIG. 3B.

Accordingly, a bitstream generated according to the fourth embodimentincludes coded differential values of the chrominance component andinformation on the selected prediction method. In addition, when thehybrid prediction method is selected and the hybrid predictor accordingto the second embodiment is used, information on a hybrid predictionmethod selected from a plurality of hybrid prediction methods is alsoincluded in the bitstream.

Image coding apparatuses and methods according to the first throughfourth embodiments of the present invention have been described.Hereinafter, an apparatus and method for decoding images coded by theabove coding methods will be described.

FIG. 6A is a block diagram of an image decoding apparatus according toan embodiment of the present invention. The image decoding apparatusincludes an entropy decoding unit 600, a dequantization/inversion unit630, a temporal-predictive compensation unit 650, a luminancespatial-predictive compensation unit 680, a chrominancespatial-predictive compensation unit 700, and an output unit 800.

FIG. 6B is a flowchart of an image decoding method according to anembodiment of the present invention. Referring to FIGS. 6A and 6B, theentropy decoding unit 600 receives a bitstream obtained by coding animage, decodes the bitstream using an entropy decoding methodcorresponding to an entropy coding method used during the coding togenerate quantized values, and outputs the quantized values to thedequantization/inversion unit 630 (S600).

The dequantization/inversion unit 630 dequantizes the quantized valuesfrom the entropy decoding unit 600 using a predetermined quantizationbit number read from a header of the bitstream and inversely transformsvalues in frequency domain to values in time domain using an inversionmethod such as inverse DCT (IDCT) corresponding to frequencytransformation used during the coding, thereby generating a differentialvalue for each pixel in an image (S630). In addition, thedequantization/inversion unit 630 determines whether the generateddifferential values are for an intra-image and outputs the differentialvalues to the luminance spatial-predictive compensation unit 680 whenthe differential values are determined as for the intra-image and to thetemporal-predictive compensation unit 650 when the differential valuesare determined as for an inter-image (S635).

The temporal-predictive compensation unit 650 generates a predictionvalue for each pixel in a current image referring to a currently decodedintra-frame image and a previously decoded inter-frame image and addseach prediction value and a corresponding differential value receivedfrom the dequantization/inversion unit 630, thereby restoring thecurrent image (S650).

Meanwhile, the luminance spatial-predictive compensation unit 680receives the differential values for a luminance component of theintra-image, generates a prediction value for each pixel of theluminance component using a prediction method read from the bitstream,and adds each prediction value and a corresponding differential valuereceived from the dequantization/inversion unit 630, thereby restoringthe luminance component of the current image (S680).

The chrominance spatial-predictive compensation unit 700 receivesdifferential values for a chrominance component of the intra-image,compensates for the differential values to restore the chrominancecomponent, and outputs the restored chrominance component to the outputunit 800 (S700).

The output unit 800 combines the restored luminance component and therestored chrominance component to output a restored image (S800).

FIG. 7A is a block diagram of the chrominance spatial-predictivecompensation unit 700 according to an embodiment of the presentinvention. FIG. 7B is a flowchart of chrominance spatial-predictivecompensation (S700) according to an embodiment of the present invention.

A prediction method determiner 720 receives the decoded differentialvalues of the chrominance component and attempts to extract information(hereinafter, referred to as a “prediction mode”) on the predictionmethod from the bitstream (S722).

When the chrominance component has been coded according to the imagecoding method and apparatus according to the first embodiment, theprediction mode does not exist. In this situation, the prediction methoddeterminer 720 calculates a variation for the current block to bedecoded, using pixel values in blocks which have been decoded prior tothe current block and are located above and on the left of the currentblock (S724). Thereafter, the prediction method determiner 720 selectsone among the prediction methods illustrated in FIGS. 5A through 5H orpredetermined prediction methods according to the variation (S726).

When the prediction mode is included in the bitstream, the predictionmethod determiner 720 extracts and analyzes the prediction mode anddetermines the prediction method used during the coding (S728).

A prediction value generator 740 generates a prediction value of eachpixel in the current block to be decoded, using previously decodedblocks according to the determined prediction method in the same manneras used to code the chrominance component, and outputs the predictionvalue to a predictive compensator 760 (S740). The prediction method usedby the prediction value generator 740 is one among the DC predictionmethod, the vertical prediction method, the horizontal predictionmethod, or the hybrid prediction method.

The predictive compensator 760 adds the prediction value to adifferential value of each corresponding pixel of the decodedchrominance component to restore the chrominance component of theintra-image (S760).

FIGS. 8A and 8B are graphs showing the test results of comparing amethod of the present invention and a method suggested by therecommendation of the JVT committee. In the present invention, thevariations dH and dV were compared with each other, only two methodsillustrated in FIGS. 5A and 5E were used, and a prediction value of eachpixel in a current block was generated using a value of a white or blacksquare-shape pixel on the same column or row as the pixel in the currentblock. The prediction method according to the present invention was usedinstead of a plane prediction method among the methods suggested by therecommendation of the JVT committee. When the present invention iscompared with the plane prediction method suggested by therecommendation of the JVT committee, the plane prediction methodrequired 323 additions, 130 multiplications, and 67 shift operations perone block while the present invention required only one conditionaloperation. Accordingly, the present invention requires just a slightamount of calculation and shows better performance than the conventionaltechnology by utilizing a statistical characteristic of a chrominancecomponent, as shown in FIGS. 8A and 8B.

In the recommendation of the JVT committee, information indicating achrominance prediction method used for each 8×8 block is coded using avariable-length code. In the present invention, a fixed-length code isused because the fixed-length code shows better compression performancethan the variable-length code when a probability of each of the DC,vertical and horizontal prediction methods and the method of the presentinvention being selected is considered. Alternatively, a predictionmethod to be used for a current block is determined using informationregarding adjacent reference blocks so that the prediction method can beused during decoding without coding the information indicating theprediction method used during coding. As described above, the presentinvention provides a simple and efficient prediction method when achrominance component of an intra-image is spatially and predictivelycoded, by using a statistical characteristic of a chrominance componentthat color does not gradually change but intermittently changes indifferent regions.

The present invention can be realized as a code which is recorded on acomputer readable recording medium and can be read by a computer. Thecomputer readable recording medium may be any type of medium on whichdata which can be read by a computer system can be recorded, forexample, a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disc, or anoptical data storage device. The present invention can also be realizedas carrier waves (for example, transmitted through Internet).Alternatively, computer readable recording media are distributed amongcomputer systems connected through a network so that the presentinvention can be realized as a code which is stored in the recordingmedia and can be read and executed in the computers.

As described above, according to the present invention, a chrominancecomponent is effectively predictively coded so that compressionefficiency is increased. In addition, since additions or multiplicationsare not required, the amount of calculation is reduced. Accordingly,time required for coding and decoding is reduced.

In the drawings and specification, preferred embodiments of theinvention have been described using specific terms but it is to beunderstood that such terms have been used only in a descriptive senseand such descriptive terms should not be construed as placing anylimitation on the scope of the invention. Accordingly, it will beapparent to those of ordinary skill in the art that various changes canbe made to the embodiments without departing from the scope and spiritof the invention. Therefore, the scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A method of processing a color component in a video, the method comprising: obtaining a residual value of the color component from a a bitstream; checking information related to intra prediction from the bitstream, for a current prediction block of the color component; generating, performed by using at least one processor, a prediction value for the current prediction block of the color component, by performing the intra prediction on the current prediction block in response to checking of the information related to the intra prediction from the bitstream; and reconstructing the current prediction block of the color component by using the prediction value and the residual value, wherein the generating the prediction value for the current prediction block of the color component comprises performing the intra prediction, based on an intra prediction mode determined from a neighboring block of the current prediction block of the color component, the neighboring block being located on at least one of a left side of the current prediction block of the color component or an upper side of the current prediction block of the color component, or based on an intra prediction mode from among a plurality of intra prediction modes as indicated by the information related to the intra prediction from the decoded bitstream, the plurality of intra prediction modes include a direct current (DC) prediction mode, a vertical prediction mode and a horizontal prediction mode. 